Cotter Pins: What Are Cotter Pins?

A cotter pin is a type of locking mechanism used to lock fasteners into place and prevent them from coming loose. Thanks to their ease of use and the simple tools required to install them, cotter pins have many different applications. Learn more about cotter pins and see how they can be used in your future projects.

What is a Cotter Pin?

In general terms, a cotter is a pin or wedge used to fix parts together lightly. A cotter pin is a pin with a flat bearing surface that passes through a hole in a fastener or machine part to keep parts together. For this reason, it may also be known as a locking pin.

How to Install a Cotter Pin

Installing a cotter pin is a relatively straightforward process. Let’s discuss it step by step:

1. Install the bolt to which you want to lock the nut.
2. Install a castle nut (or castellated nut) where you need it on the bolt. The nut must be castellated (have grooves or holes) to allow for positioning the pin and locking it in place.
3. Leave the castle nut in place, or mark the bolt where you’d like to drill a hole for the cotter pin.
4. Use a metal punch to make a starting point for the hole you’re about to drill. Then, use a drill and bit to drill straight through the center of the bolt.
5. If necessary, use a small file to smooth the edges of the hole. Ensure that you remove any metal filings.
6. Insert the cotter pin, and use a pair of pliers to bend the two ends of the pin to the sides where they’ll lock the pin (and, therefore, the nut) in place.

Are Cotter Pins Necessary?

Cotter pins are not necessary for every project, but their use depends significantly on the project at hand. In some cases, including the front wheel design on many motorcycles, a cotter pin is absolutely essential since it is the only reliable wheel that locks the fastener in place.

Measuring Cotter Pins

Understanding which cotter pin to buy is essential, but how are cotter pins measured? 

The length of the pin is the length of the shortest pin, starting at the base of the pin’s round head and extending to the end of the shortest pin. The pin’s diameter is the width of the two pins at the base of the cotter, not the diameter of the pin’s head.

So, if you’re wondering how to measure a cotter pin, simply use a caliper to measure the abovementioned aspects of the pin.

Are Cotter Pins Reusable?

Cotter pins should not generally be reused. While they typically consist of strong and pliable materials, they’re typically fairly stiff and unyielding once deformed. This creates a reliable locking mechanism that can be relied on indefinitely.

If you remove a cotter pin, the bends of the pin become more brittle. So, if you reuse a used cotter pin, it significantly increases the likelihood of that pin failing in the future.

How to Remove a Cotter Pin

Removing a cotter pin is as simple as using pliers to straighten out the bent pin. Once you’ve bent the pin back into place and straightened them enough to fit through the hole in the bolt, you can simply grab the head of the pin and pull it back through the bolt.

Why Use Cotter Pins?

The main reason for using cotter pins is that they offer a reliable locking mechanism that may be considerably more trustworthy than some other locking options. They’re also simple, quick, and easy to install and replace, making them an ideal quick fix for locking installation.

However, the main benefit to consider is that they prevent nuts from coming loose when exposed to vibrations. Cotter pins essentially eliminate the risk of a nut coming loose, except in a case where the force applied is enough to shear through the pin.

Types of Cotter Pins

There are several different types of cotter pins, with different purposes and applications. These include the following:

• Hair Pin Cotter Pins - These are some of the most common types of pins, characterized by easy installation and removal. 
• Wire Snap Cotter Pins - These pins are commonly used in shafts or clevis pins to keep components aligned in place.
• Hammer Set Cotter Pins - As the name suggests, you tap these pins with a hammer to install them. They are intended as a temporary holding measure.
• Wedge Lock Cotter Pins - These pins are also installed with a hammer but are intended as a permanent solution. Tapping them with a hammer after installation wedges the prongs open to lock the pin in place

Cotter Pins vs. Split Pins

Split pins are sometimes considered an interesting alternative to cotter pins. However, they are actually exactly the same thing.

Cotter pins consist of a type of mild steel. You insert them into the bolt’s hole, then bend the pins into a different shape to keep the pin locked in place, and prevent the nut from falling out.

Some people claim that split pins consist of a higher-grade, harder steel. However, these are really just different types of cotter pins made from different grades of steel.

Now that you understand cotter pins, check what other fasteners can be used for your project, and place your order!

Frequently Asked Questions About Cotter Pins

What are cotter pins used for?

The cotter pin is essentially a locking mechanism used to pin fasteners like nuts in place on bolts, shafts, or other similar fastening mechanisms. Unless you use a clevis pin, which comes premade with an existing hole, you will need to drill a hole through the bolt to use a cotter pin. 

How does a cotter pin work?

A cotter pin, essentially, is a pin-shaped wedge that opens up once you’ve placed the pin through a hole in your bolt or fastener. 

The larger, rounded head on the pin prevents it from slipping all the way through the hole. Once the “legs” of the pin have been opened, they prevent the pin from traveling back through the hole. They’re generally paired with a castle nut that locks onto the cotter pin to keep everything in place.